1. Field of Invention
The present invention relates to a method of manufacturing mask read-only-memory (ROM). More particularly, the present invention relates to a method that uses a damascene process to manufacture the bit lines of a mask ROM.
2. Description of Related Art
The bit lines of most flat cell mask ROM are formed by implanting ions into a substrate to form a buried layer. However, as the level of integration increases, resistance of these buried bit lines also increases, and hence the ultimate operation speed of the mask ROM is limited. The reason for this is that even by doping the substrate with the highest possible concentration of N-type ions, the ultimate resistivity can only reduced to about 70 to 80 .OMEGA./.quadrature.. If, for the sake of decreasing the resistivity of the buried lines, the ion concentration in the doping operation is further increased, punch through between neighboring memory cell bit lines may happen.